Devices and systems are known for enabling secure, keyless access control including vehicular access control. An authorized person possesses a radio frequency (RF) device, for example an RF identification (ID) tag embedded in an identification badge or credit card-type medium to enable access. Building access systems may rely upon swiping the badge or card through an energizer/reader which interrogates the RFID tag and verifies that certain information carried by the RFID tag meets the security requirements to allow access. Similarly, a vehicular access control system may rely upon door handle activation to initiate an interrogation and verification. If the door handle operator possesses a key, a key fob, badge, card or the like including an RFID tag with information meeting the security requirements, the door locks are actuated to provide access to the vehicle. Additional interrogation may enable operative functionality of the vehicle systems, including keyless push-button starting, in similar fashion. Related RF based systems are in use for authorizing purchases where small RF devices intended primarily for coupling to a key chain are brought into proximity of an RF energizer/reader associated with a dispensing mechanism for the product being purchased. In such systems, the RF device stores information related to the user's credit card account for completing the transaction. Other systems are known for automating the collection process on toll roads where an RF device carried by a vehicle is interrogated and communicates information back to an RF energizer/reader associated with certain designated toll collection booths.
Passive wireless devices (i.e. those requiring interrogation before transmission) may be categorized as backscatter type which reflects back a characteristic signal to the energizer/reader subsequent to the energizer/reader sending an interrogation signal. The energizer/reader can identify and distinguish unique backscatter signatures. This type of wireless system is generally a lowest cost approach. Another type of wireless system receives the interrogation signal, rectifies the RF energy to a DC source and uses the DC energy to send information stored in sensor memory. Another type has its own power source—commonly a battery. The battery may be completely independent of the RF energizer/reader energy or alternatively recharged by the RF energizer/reader signal. In both of these latter wireless systems, the RF signal serves to interrogate or wake up the wireless device. Such wireless devices use the interrogation signal for initiating the provision of stored information back to the energizer/reader.
Electromagnetic emissions are regulated. Such regulations limit the RF energy emitted by an energizer/reader in the frequency band where RF devices operate. Current United States regulations limit such energy emissions to 4 watts. Similar regulations exist in other countries around the world. Such energy restrictions effectively limit the effective range between the energizer/reader and the RF device.
The communication links of an RF system including an energizer/reader and remote RF device are not balanced. That is to say, the energizer/reader emits a relatively greater amount of RF energy than does the RF device. The energizer/reader must be located close enough to the RF device to effectively energize and/or interrogate the RF device to effect the desired return response from the device. If too far away from the energizer/reader, or in an area of attenuated RF signal, the RF responsive device will not receive enough energy to backscatter, to appropriately energize and effect a response, or to distinguish the energizer/reader emission as an interrogation looking for a response.
RF device communication systems are generally limited due to regulations. Beginning with the forward communication link power limitation, losses in communicating to the RF device include propagation losses, RF device antenna inefficiencies and losses in the energy and signal conversions for signal processing and energy utilization at the RF device. And, these same types of inefficiencies are repeated within the RF device in the reverse communication link from the RF device to the energizer/reader. The remaining reverse communication link, which includes the energizer/reader front end and digital signal processing, however, is more robust being characterized by effective noise immunity and the ability to detect relatively low level signals. Therefore, RF device communication systems are generally more limited with respect to the forward communication link than with respect to the reverse communication link.
Effective systems therefore require designed proximity and orientation between the energizer/reader and the RF device to ensure proper operation. In conventional applications, the energizer/reader and the RF device are dynamically manipulated such that the proximity and orientation requirements for effecting the desired interaction are met. This may include, for example, manipulating the RF device into an appropriate position relative to a static energizer/reader, manipulating the energizer/reader into an appropriate position relative to a static RF device or combinationally manipulating the RF device and the energizer/reader into an appropriate cooperative orientation. Such systems are used, for example, in inventory management control and product tracking. One such system may convey goods carrying an RF device along a defined path which includes an energizer reader oriented such that the conveyance brings the RF device within the proper proximity and orientation of the energizer/reader to effect the desired operation. Another such system may make use of a portable or maneuverable energizer/reader for spot checking material at various ad-hoc locations within a distribution chain—for example at a receiving dock, a warehouse or just about anywhere within a distribution chain—by maneuvering the energizer/reader within the proper proximity and orientation of the RF device to effect the desired operation. These systems tend to be characterized by relatively narrow interrogation fields relative to the energizer/reader within which the RF device can be made to, or is reasonably anticipated to, temporally but sufficiently reside to effect the desired interrogation. These systems are not generally concerned with wide range interrogation fields which, in fact, might undesirably initiate unintentional interrogations of RF devices away from the intended RF device thereby complicating energizer/reader tasks associated with intelligibly discerning return signals.